WO1998050670A1

WO1998050670A1 - Method for drilling in the earth, and vessel to be used for this method

Info

Publication number: WO1998050670A1
Application number: PCT/NL1998/000244
Authority: WO
Inventors: Anne Klaas De Groot
Original assignee: Itrec BV
Current assignee: Huisman Equipment BV
Priority date: 1997-05-07
Filing date: 1998-05-04
Publication date: 1998-11-12
Anticipated expiration: 1999-11-07
Also published as: BR9809232A; AU7351498A; GB9926158D0; NO995359D0; NL1005995C2; GB2340865B; GB2340865A; NO995359L

Abstract

The present invention relates to a method for drilling in the earth, for example for oil or gas, from a vessel, a drill head attached to a drill string being used for the drilling; the top end of the drill string being attached, with the aid of hoisting means, to a derrick or drilling mast which is placed on the vessel; the hoisting means being provided with coupling means for coupling on the drill string and with spring means for damping movements of the vessel with respect to the drill string as a result of swell and wave action on the vessel; and the drill string being essentially surrounded between the vessel and the bed by a riser which is attached to the bed, in which method the top end of the drill string or the coupling means is connected, with the aid of tensioning means, to the top end of the riser, it being possible to move the top end of the drill string, with the aid of the tensioning means, counter to the action of the spring means.

Description

Title: Method for drilling in the earth, and vessel to be used for this method.

The present invention firstly relates to a method for drilling in the earth, for example for oil or gas, from a vessel, a drill head attached to a drill string being used for the drilling, the top end of the drill string being attached, with the aid of hoisting, means, to a derrick or drilling mast which is placed on the vessel, the hoisting means being provided with coupling means for coupling on the drill string and with spring means for damping movements of the vessel with respect to the drill string as a result of swell and wave action on the vessel, and the drill string being essentially surrounded between the vessel and the bed by a riser which is attached to the bed.

According to the prior art, during drilling, a drill string, with a drill head at its end, is moved away from a derrick positioned on the drilling vessel and towards the earth. During drilling, the drilling vessel moves with respect to the earth under the influence of waves and the wind. Consequently, the vessel therefore also moves in the vertical direction with respect to the drill string which has been positioned in the earth. That end of the drill string which is directed away from the earth is consequently not fixedly connected to the derrick of the vessel, but rather is attached to the derrick via a spring structure. As a result of this structure, a vertical movement of the vessel is not translated into a corresponding vertical displacement of the drill string. The spring structure by means of which the drill string is coupled to the derrick is also known as the drill string compensator.

A drill string compensator of this nature is known, inter alia, from the General Catalogue 1984-1985 of the NL Industries Inc. company. This known drill string compensator comprises, inter alia, a main frame, on which are positioned the so- called travelling block and a hook plate, the hook plate being attached to the underside of the main frame with the aid of spring means, which are formed by a hydropneumatic suspension. The main frame is attached displaceably to a guide which is attached in the derrick. The main frame is connected to the tower with the aid of a hoisting cable. The hoisting cable is guided on the one hand through stationary blocks which are attached to the mast and on the other hand by the travelling block on the main frame. With the aid of the hoisting cable, the main frame can thus be moved with respect to the mast or the tower, along the guide.

The hook plate is also attached displaceably to the guide, and the hook plate moves together with the main frame. With the aid of coupling means, that free end of the drill string which faces away from the earth is attached to the hook plate.

If the vessel moves in the vertical direction with respect to the drill string during drilling, the position of the main frame is stationary with respect to the guide, while the hook plate, supporting the full weight of the drill string, can move along the guide with respect to the main frame, owing to the presence of the hydropneumatic suspension. This means that the hydropneumatic suspension ensures that the movements which the drill string forces on the hook plate are transmitted to the main frame in a damped manner. A significant drawback of the method according to the prior art is that the drill string is attached to a hook plate which is attached to the main frame by a very slack hydropneumatic spring. The spring has to be slack, because a tighter spring would result in greater variations of the pressure on the drill head in the event of vertical movements of the ship. In order to be able to form a slack hydropneumatic spring, it is necessary to connect the hydropneumatic cylinders of the suspension to a relatively large volume of compressed air. This requires the use of a relatively large, expensive and heavy compressed-air installation. Moreover, an installation of this nature takes up a considerable amount of space.

The object of the present invention is to provide a method in which drilling takes place with a drill string compensator which is able to keep the pressure on the drill head more constant than drill string compensators according to the prior art.

Another object is to provide an installation for which a smaller volume of compressed air is sufficient.

These objects are achieved by the fact that the top end of the drill string or the coupling means is connected, with the aid of tensioning means, to the top end of the riser, it being possible to move the top end of the drill string, with the aid of the tensioning means, counter to the action of the spring means.

The advantage of this method is that it produces a coupling between the drill string and the riser. Since the riser itself is attached to the seabed, as a result the drill string is also fixed with respect to the earth. If the vessel now moves in the vertical direction with respect to the seabed, the pressure on the drill head does not change. The ultimate advantageous effect of this measure is that the drilling pressure remains relatively constant, even in the event of vertical movements of the ship with respect to the seabed. Since the demands which have to be placed on the spring characteristic of the drilling compensator can be less severe, a relatively stiff spring is also sufficient in this method. The relatively large, heavy, expensive and space- consuming compressed-air installation is made superfluous by this design. Since the spring attached to the derrick or drilling mast produces a force on the drill string which acts in the opposite direction to the force of gravity acting on the drill string, with the result that the drill string is completely or partially lifted up, the inherent weight of the drill string does not rest entirely on the drill head, or does not do so at all. With the aid of the tensioning means, the drill string lifted up by the spring of the drill string compensator can be moved towards the earth, counter to the action of the spring. This means that the initial force exerted on the drill head can be adjusted with the aid of the spring and the tensioning means together.

The present invention relates not only to a method but also to a device for carrying out the said method. The said device comprises a drilling vessel for drilling in the earth, for example for oil or gas, from the said vessel, with the aid of a drill head which is attached to a drill string, the drill string, during the drilling process, being essentially surrounded between the vessel and the bed by a riser which is attached to the bed, comprising: a floating body, a derrick or drilling mast which is placed on the said floating body and is equipped with hoisting means for attaching a drill string to the said derrick or drilling mast, the said hoisting means comprising at least spring means for damping movements of the floating body with respect to the drill string as a result of swell or wave action.

The vessel according to the present invention is characterized in that the drilling vessel is provided with tensioning means in order to connect the top end of the drill string to the top end of the riser. Moreover, it is possible according to the invention for the top end of the drill string to be connected to the floating body using the tensioning means.

American patent 4,085,509 has disclosed a device for measuring the penetration rate of a drill pipe into a source. In this known device, a line or cable, which is held under tension and is guided towards the vessel, is attached to both the top end of the drill string and the top end of the riser. These lines are guided over measurement wheels. If the drilling vessel moves with respect to the seabed, the measurement wheels independently record the movement of the vessel with respect to the seabed and the movement of the drill string with respect to the seabed. By combining the measurement data, the penetration rate of the drill pipe into the earth can be worked out. In the known device, the lines or cables are intended only as measurement lines. The lines cannot be used to exert a tensioning force on the pipe in order to be able to compensate for movements of the vessel as a result of waves, swell and wind. Moreover, the measurement cable which is attached to the top end of the drill string, as can be seen from Figure 1 of the patent, is guided upwards from the drill string. Therefore, in contrast to the tensioning means according to the present invention, the cable cannot be used to exert a force on the drill string in the direction of the seabed.

According to an advantageous embodiment of the invention, it is possible that the spring means are formed by a drill string compensator, which comprises the following parts: a main frame which is attached displaceably to the derrick or drilling mast, it being possible to displace the said main frame with respect to the derrick or drilling mast with the aid of hoisting means, a hook plate, which is attached to the said derrick or drilling mast such that it can be displaced with respect to the derrick or drilling mast, the hook plate being provided with coupling means for coupling the drill string fixedly to the hook plate, and at least one spring arranged between the said main frame and the said hook plate, in which the tensioning means are attached to the hook plate. In this case, it is advantageous for the tensioning means to comprise a winch.

The drilling vessel according to the invention has a number of advantages. Firstly, the winch can be used, prior to drilling, for positioning large, relatively heavy objects on the seabed in a safe manner. For example, it is very difficult to use the drill string compensators which are employed in the prior art to place a blow-out preventer (B.O.P.) in a controlled manner on a baseplate which has been laid on the seabed without damaging the B.O.P.

When placing the B.O.P. on the seabed, according to the method in accordance with the prior art the B.O.P. is attached to the hook plate. If the vessel move^'s in the vertical direction with respect to the seabed while the B.O.P. is being put in place, the hook plate also moves with respect to the seabed, and therefore the B.O.P. attached to the hook plate also moves with respect to the seabed. This can cause the landing of the B.O.P. to be hard as a result of the rocking movement of the vessel. According to the method in accordance with the present invention, it is possible to position the B.O.P., and other heavy objects, on a baseplate or on the seabed in a controlled manner. The reason for this is that the hook plate can be moved independently of the main frame with respect to the vessel. This can be achieved with the aid of tensioning means which are attached between the hook plate and the vessel. These tensioning means may, for example, be designed in the form of a winch. In that case, the wire of the winch is fastened, for example, to the deck of the ship. This contrasts with the drilling operation, during which the wire of the winch is connected to the top of the riser. Fastening the wire of the winch to the vessel itself has the advantage that when the B.O.P. is being put in place, the weight of this B.O.P. can be suspended from the hook plate, while the positioning of the hook plate with respect to the ship can be controlled by means of the winch positioned on the hook plate. Any movements of the vessel with respect to the seabed can be compensated for with the aid of the said winch, so that the B.O.P. in principle does not undergo any rocking movements with respect to the seabed and can be placed on the seabed in a controlled manner.

Secondly, the winch attached to the hook plate can, as already indicated above, be used for fixing the drill string to the riser, and hence to the seabed, in the vertical direction. A winch provides a relatively inexpensive, easily controllable and effective design of the desired tensioning means. A known problem with the drill string compensators according to the prior art is that they have to be attached to a guide arranged in the derrick. Since both the main frame and hook plate have to be attached displaceably to the said guide, it is important that the said guide is accurately aligned. The said guide has to remain straight even when considerable forces are exerted on it. The guide therefore has to be attached to the framework structure of the derrick using a large number of adjustment bars. Aligning and attaching the guide to the derrick is time-consuming and consequently expensive.

In the present invention, it is sought to make the said derrick or drilling mast essentially in the form of a case. It is advantageous here for the drill string compensator to be attached to the projecting plate such that it can be displaced with the aid of guide wheels. This has the considerable advantage that the planar, rectangular plate, which forms a direct component of the derrick, can be used to guide the drill string compensator. Since the plate is planar and rigid, the plate, and consequently the guide, do not have to be arranged independently of the derrick. Moreover, the guide will remain stable and straight even under a very heavy load.

Another drawback of the drill string compensators according to the prior art is that the suspension between the main frame and the hook plate comprises, in addition to a number of cylinders, a chain transmission. Since the drill string compensators according to the prior art have to be positioned in the derrick, it is the case that the larger the drilling compensator has to be, the larger the derrick also has to be. It is clear that as the size of the derrick increases its cost also grows. The chain transmission is used to prevent the need to use a long cylinder in order to increase the stroke length. The chain transmission significantly increases the stroke length of the relatively short cylinders used. However, the use of the chain transmission also makes the design more complex.

By using a drilling mast in the form of a case, the drill string compensator according to the present invention can be attached essentially to the outside of this mast.

The drill string compensator according to the present invention is improved still further by the fact that the said spring between the main frame and the hook plate comprises hydropneumatic cylinders, which are positioned essentially parallel to the derrick or drilling mast, in which case at least one hydropneumatic cylinder, which is attached to the main frame, is connected so that it acts in series with a hydropneumatic cylinder which is attached to the hook plate.

This has the advantage that the drill string compensator according to the present invention can be composed of a limited number of relatively inexpensive components. Relatively expensive components, such as for example chain transmissions, which moreover are susceptible to wear, are not required in this design.

The present invention will be explained with reference to the following figures, in which: Figure 1 shows a side view of a drilling vessel with a drill string underneath it running down into the seabed.

Figure 2 shows a diagrammatic depiction of a drill string compensator according to the prior art. Figure 3 shows a detail of the drilling mast of a drilling vessel with the drill string compensator according to the present invention attached to it.

Figure 4 shows a diagrammatic depiction of the action of the drill string compensator according to the present invention.

Figure 1 shows a side view of a drilling vessel 1 , with a drill string 5 attached thereto and running down into the seabed 7. The drilling vessel 1 comprises a floating body 2 with a drilling mast 3 attached thereto. In the region of the top end of the drilling mast 3, the drill string 5 is attached to the drilling mast 3 with the aid of a drill string compensator 4. The drill string 5 comprises a large number of pipes coupled together and extends from the top side of the drilling mast 3 as far as the drill head 6 which is attached to the free end of the drill string 5. During drilling, the drill string 5 disappears deeper and deeper into the seabed 7, via the riser 10. Drill string pipes are added to the top of the drill string 5 one by one from the vessel 1 , the length of the drill string 5 thus increasing.

The drill string 5 cannot immediately be introduced into the seabed 7 from the drilling vessel 1. Prior to the drilling process, a baseplate 8 is positioned on the bed of the sea 7. A blow-out preventer (B.O.P.) 9 is then placed on this baseplate 8. During drilling, this B.O.P. 9 prevents highly pressurized gas and/or liquid from being able to emerge in an uncontrolled manner. A so-called riser 10 is arranged between this B.O.P. 9 and the drilling vessel 1. That free end of the riser 10 which is directed away from the seabed 7 opens out into a riser top part 20. The design of this riser top part 20 can be seen clearly in Figure 3.

In order to protect the drill hole and to facilitate the return flow of the drilling fluid to the vessel 1 , the drill hole is provided with a casing 11. The top section of the casing 11, directly below the baseplate 8, is moreover reinforced with the aid of a cement jacket 12. At least part of the casing 11 is surrounded by the cement 12.

The problem underlying the present invention can be outlined with reference to Figure 1. During drilling from the drilling vessel 1 , the complication could arise where the drilling vessel 1 can perform vertical movements with respect to the seabed 7. These movements are caused, for example, as a result of waves or the wind. Since the drill string 5 is coupled to the drilling vessel 1 via the drill string compensator 4, the force with which the drill string 5 rests in the drill hole varies as a result of the movements of the ship 1. This is very undesirable, since the drilling pressure on the underside of the drill string 5 on the drill head 6 is consequently directly dependent on the movements of the drilling vessel 1 itself. In order to prevent the drill string 5 from following the vertical movements of the drilling vessel 1, in the prior art the top end of the drill string 5 is attached to the drilling vessel 1 via a drill string compensator containing a very slack spring. A diagrammatic illustration of a drilling compensator according to the prior art can be seen in Figure 2. This compensator comprises a main frame 21 , which is attached to the derrick or drilling mast 3 by means of a cable block 26. The main frame 21 can move in the vertical direction along a guide in the tower 3. A hook plate 24 is attached to the main frame 21 by means of a spring 27, which is formed, for example, by hydropneumatic cylinders. The top end of the drill string 5 is attached to this hook plate 24. The spring 27 will absorb all the movements which the drilling vessel 1 performs with respect to the seabed 7. It is important that the spring 27 in the drilling compensators according to the prior art is slack, since the stiffness of the spring 27 determines the magnitude of the variations in force on the drill head caused by the vertical movements of the ship. Drill string compensators according to the prior art generally comprise hydropneumatic cylinders. In order to be able to construct a slack spring by means of these cylinders, the cylinders have to be connected to a relatively large, expensive and heavy compressed-air installation.

Figure 3 is a side view of the drilling mast 3 according to the present invention, with the drill string compensator 4 according to the present invention attached thereto. Just like the drilling compensators according to the prior art, the drill string compensator 4 has a main frame 21. The said main frame 21 is movably attached to the drilling mast 3 with the aid of guide arms 32. The main frame 21 can be moved in the vertical direction along a guide. In the drilling mast 3 this guide is formed by longitudinal sides of the plate 33. It can be seen in Figure 3 that the mast 3 is constructed as a case. It is not necessary for the longitudinal sides of the guide plate 33 to project in order to guide the drill string compensator 4. An important advantage of the use of a case structure is that the stiffness which is inherently present in the case is utilized for the guidance. The vertical position of the main frame 21 with respect to the drilling mast 3 can be controlled with the aid of the cable blocks 36 which are positioned on the main frame 21. The drill string compensator 4 according to the present invention furthermore comprises a hook plate 24, to which the coupling means 38 are connected in order to attach the top side of the drill string 5 to the drilling compensator 4. During drilling, the drill string 5 projects from these coupling means 38, via the turntable 30, and through the top piece of the riser 20 towards the drill hole.

The hook plate 24 is movably attached to the drilling mast 3 via guide arms 35 and is connected to the main frame 21 via hydropneumatic cylinders 37. These cylinders 37 are placed in series and are coupled at the coupling point 41.

According to the present invention, a characterizing feature of the drill string compensator 4 is that it is provided with a winch 42 which is attached to the hook plate 24. The cable attached to this winch 42 is connected to the end of the riser 20. Since the bottom of the riser 20 is connected, via the B.O.P. 9 and the baseplate 8, to the seabed 7 (see Figure 1), the addition of the winch 42 to the hook plate 24 means that this hook plate 24 is mechanically connected to the seabed 7. The major advantage of this is that the spring characteristic of the spring which is formed, inter alia, by the cylinders 37 can be selected to be relatively rigid. Since the drill string 5 is mechanically coupled to the seabed 7, the drill string 5 will not follow the movements of the vessel 1 even in the event of a relatively rigid spring characteristic of the spring formed by the cylinders 37. A relatively small compressed-air installation is sufficient to be able to form a spring with a relatively rigid spring characteristic. A second important plus point of the drill string compensator 4 according to the present invention is that the spring characteristic can be selected to be such that the entire weight of the drill string 5 is in principle suspended from the drilling mast 3. This means that the inherent weight of the drill string 5 is not pressing on the drill head, but rather the entire drill string 5 is pulled loose from the drill hole by means of the spring which is formed by the cylinders 37. In order still to be able to drill, the hook plate 24 is pulled towards the top of the riser 20 with the aid of the winch 42. This pulling movement pushes the drill string 5 vertically downwards. The drill head 6 is pulled into the ground at a defined rate by the winch 42. The force which is required for this is measured with the aid of a sensor on the winch 42. This measured force can be used to adjust the speed at which the drill head 6 is pulled downwards with the aid of control means, optionally automatically. As a result, it is therefore possible to control the rate of progress of the drill head 6 at the end of the drill hole.

For the sake of clarity, the functioning of the drilling compensator 4 according to the present invention is depicted diagrammatically in Figure 4. In Figure 4, it can be seen that by adding the winch 42 to the hook plate 24, the hook plate 24 is mechanically connected to the seabed 7. It is conceivable that something else could be used instead of a winch 42 for this coupling. The essential feature is that there is a connection between the hook plate 24 and the riser, which is such that the position of the hook plate 24 can be adjusted with respect to the top of the riser 20. A winch 42 is merely one of the possibilities of achieving this.

Claims

CLAIMS 1. Method for drilling in the earth (7), for example for oil or gas, from a vessel (1), a drill head (6) attached to a drill string (5) being used for the drilling; the top end of the drill string (5) being attached, with the aid of hoisting means, to a derrick or drilling mast (3) which is placed on the vessel (1); the hoisting means being provided with coupling means (38) for coupling on the drill string (5) and with spring means (4) for damping movements of the vessel (1) with respect to the drill string (5) as a result of swell and wave action on the vessel (1); and the drill string (5) being essentially surrounded between the vessel (1) and the bed (7) by a riser (10) which is attached to the bed (7), characterized in that the top end of the drill string (5) or the coupling means (38) is connected, with the aid of tensioning means (42), to the top end (20) of the riser (10), it being possible to move the top end of the drill string (5), with the aid of the tensioning means (42), counter to the action of the spring means (4).

2. Drilling vessel (1) for drilling in the earth (7), for example for oil or gas, from the said vessel (1), with the aid of a drill head (6) which is attached to a drill string (5), the drill string (5), during the drilling process, being essentially surrounded between the vessel (1) and the bed (7) by a riser (10) which is attached to the bed (7), comprising: a floating body (2), a derrick or drilling mast (3) which is placed on the said floating body (2) and is equipped with hoisting means for attaching a drill string (5) to the said derrick or drilling mast (3), the said hoisting means comprising at least spring means (4) for damping movements of the floating body (2) with respect to the drill string (5) as a result of swell or wave action, characterized in that the drilling vessel (1) is provided with tensioning means (42) in order to connect the top end of the drill string (5) to the top end (20) of the riser (10).

3. Drilling vessel (1) according to Claim 2, characterized in that the top end of the drill string (5) can be connected to the floating body (2) using the tensioning means (42).

4. Drilling vessel (1) according to Claim 2 or 3, in which the spring means (4) are formed by a drill string compensator, which comprises the following parts: a main frame (21) which is attached displaceably to the derrick or drilling mast (3), it being possible to displace the said main frame (21) with respect to the derrick or drilling mast (3) with the aid of hoisting means, a hook plate (24), which is attached to the said derrick or drilling mast (3) such that it can be displaced with respect to the derrick or drilling mast (3), the hook plate (24) being provided with coupling means (38) for coupling the drill string (5) fixedly to the hook plate (24), and - at least one spring arranged between the said main frame (21) and the said hook plate (24), characterized in that the tensioning means (42) are attached to the hook plate (24).

5. Drilling vessel (1) according to Claim 2, 3 or 4, characterized in that the tensioning means (42) comprise a winch.

6. Drilling vessel (1) according to one of Claims 2 to 5, characterized in that the derrick or drilling mast (3) is essentially in the form of a case.

7. Drilling vessel (1) according to Claim 6, characterized in that one of the walls (33) of the case projects in the horizontal direction with respect to the case.

8. Drilling vessel (1) according to Claim 7, characterized in that the drill string compensator (4) is attached to the projecting plate (33) such that it can be displaced with the aid of guide wheels (32).

9. Drilling vessel according to one of the preceding claims, characterized in that the spring between the main frame and the hook plate comprises hydropneumatic cylinders, which are positioned essentially parallel to the derrick or drilling mast, characterized in that at least one hydropneumatic cylinder (37), which is attached to the main frame (21), is connected so that it acts in series with a hydropneumatic cylinder (37) which is attached to the hook plate (24).